1. Field of the Invention
The present invention relates to a liquid crystal display apparatus and a method for lighting a backlight thereof. More specifically, the present invention relates to a liquid crystal display apparatus that can reduce noises so as to improve a quality of display and a method for lighting a backlight thereof.
2. Description of the Related Art
A liquid crystal display apparatus illuminates a liquid crystal panel using a backlight so as to realize an image by the difference of partial transparency of the liquid crystal panel. Conventionally, the backlight is driven by an oscillation wave having a single frequency. The phase of the oscillation wave is synchronized in the entire of the backlight (see Japanese Patent Laid-open Publication No. 11-3039 and No. 4-241320).
FIG. 1 is a block diagram showing a conventional apparatus for lighting the backlight. In FIG. 1, a driver circuit 41 drives lamps 45 and 46, while a driver circuit 42 drives lamps 47 and 48. In the same way, a driver circuit 44 drives lamps 51 and 52. Lamps 47 through 52 are driven in the same period and in synchronization with that of oscillation waves of the lamps 45 and 46. This configuration enables to light all lamps in the same period and in synchronization with each other. FIG. 2 is a block diagram showing another conventional apparatus for lighting the backlight. In the apparatus for lighting the backlight as shown in FIG. 2, a single driver circuit drives one lamp. In this configuration too, the lamps 45 through 62 are driven by the oscillation waves having the same period and are in synchronization with each other. Thus, all lamps are lighted in the same period and in synchronization with each other.
Next, the reason why all lamps should be driven in the same period and in synchronization with each other will be explained. If the oscillation waves for driving the lamps are not synchronized with each other, plural lamp oscillations will be interfered to each other, and a large interference noise will be generated periodically. This will appear and disappear on the display screen periodically to be a noise, and generates a so-called xe2x80x9cripple phenomenonxe2x80x9d. In order to avoid this, the oscillation waveforms of all lamps are oscillated in the same period and in synchronization with each other as shown in FIG. 3. If the oscillation waves of the lamps are not synchronized, the composed waveform varies along time, and a horizontal synchronization frequency of a video signal that generates the ripple cannot be specified, so that a ripple noise will be generated regardless of a video input mode. Furthermore, in the conventional apparatus for lighting a backlight, the lamp oscillation frequency is selected so that the oscillation wave does not interfere the horizontal synchronization signal.
However, the conventional apparatus for lighting a backlight has a following problem. In a multiscanning monitor, plural horizontal synchronization frequencies are used, while the lamp oscillation frequency is fixed. Therefore, the interference of the horizontal synchronization signal with the lamp oscillation wave cannot be avoided. Far this reason, the lamp oscillation frequency is selected so that only one frequency that is mainly used is not interfered. Accordingly, some of the horizontal synchronization frequencies can be always interfered, resulting in a generation of the xe2x80x9cripple phenomenonxe2x80x9d.
In addition, because of the synchronization, when the horizontal synchronization signal of the video signal is interfered with the oscillation frequency of the inverter, the ripple noise on the display further grow with an increasing number of the lamps.
An object of the present invention is to provide a liquid crystal display apparatus with improved quality of display and a method for lighting the backlight thereof by eliminating the ripple noise due to the interference between the oscillation wave of the backlight and the horizontal synchronization signal in a multiscanning monitor so as to educe the noise in a liquid crystal panel.
A liquid crystal display apparatus according to the present invention includes a first backlight and a second backlight which are driven simultaneously. The second backlight has an oscillation wave whose phase is inverted with respect to an oscillation wave of the first backlight.
A method for lighting a backlight of a liquid crystal display apparatus according to the present invention includes the steps of lighting the first backlight and lighting the second backlight simultaneously with the first backlight by inverting the phase of the oscillation waveform with respect to the oscillation waveform of the first backlight.
According to the present invention, the sine wave noise can be canceled since there are the first and the second backlight whose phases of the oscillation waveforms are opposite to each other. Therefore, the generation of the ripple phenomenon can be suppressed, and the noise due to the ripple phenomenon can be eliminated, so that a liquid crystal display apparatus with improved quality of display can be provided.